1. Field of the Invention
The present invention relates to a cellulose dope. More particularly, the present invention is concerned with a cellulose dope comprising an aqueous sodium hydroxide solution and cellulose dissolved in the solution, wherein the cellulose dope has a sodium hydroxide concentration (CNa) of from 6.5 to 11% by weight, a cellulose content (Cc) of from 5 to 10% by weight, a viscosity average degree of polymerization (DPc) of the cellulose of from 350 to 1,200 and a cellulose dissolution ratio (Sc) of 99.0% by weight or more, and wherein the zero-shear viscosity (η0) of the cellulose dope, the cellulose content (Cc) of the cellulose dope, and the viscosity average degree of polymerization of the cellulose in the cellulose dope satisfy a specific relationship. The present invention is also concerned with a method for producing the cellulose dope. The cellulose dope of the present invention is stable and free from gelation even when the cellulose dope has a high content of cellulose having a high degree of polymerization. By the use of the cellulose dope of the present invention, it becomes possible to produce, without using a large amount of a poisonous substance, a regenerated cellulose shaped article (such as a fiber or a film) which has a mechanical strength sufficient for practical use. Therefore, the cellulose dope of the present invention is commercially very advantageous. Further, the present invention is also concerned with a cellulose shaped article obtained by subjecting the cellulose dope to coagulation treatment, and a cellulose slurry used for producing the cellulose dope.
2. Prior Art
Cellulose is a polysaccharide having a linear chain comprising a number of D-glucose (D-glucopyranose) molecules, wherein the mutually adjacent molecules are bonded to each other through β-1,4-bond. Cellulose is a very important substance in industries, which has been used in various fields.
An important example of the usage of cellulose is the use as a material for the so-called regenerated cellulose shaped article. A regenerated cellulose shaped article can be produced by a method which comprises dissolving cellulose in an appropriate solvent to obtain a cellulose solution, and subjecting the obtained cellulose solution to an appropriate chemical treatment, thereby coagulating the cellulose solution. However, cellulose is insoluble in any of water and organic solvents generally used in the art (e.g., alcohols). Therefore, for producing a cellulose solution for use in the production of a regenerated cellulose shaped article (hereinafter, such a solution is frequently referred to as a “cellulose dope”), a special solvent and a special chemical treatment for cellulose are needed.
Most representative examples of regenerated cellulose shaped articles include a fiber and a film. A fiber of regenerated cellulose is called a “rayon”, and a film of regenerated cellulose is called a “cellophane”. Both of them are very widely used. Further, in some cases, regenerated cellulose shaped articles are used in a particulate form or used in the form of a coating formed on a substrate.
Further, by coagulating a cellulose dope having incorporated therein various edible components, such as a polysaccharide, a polypeptide and a natural resin, an edible shaped article containing the cellulose as a substrate can be obtained (hereinafter, such an edible shaped article is frequently referred to as an “edible cellulose article”).
At present, the method for the production of regenerated cellulose shaped articles which is commercially practiced is performed mainly by either of two processes, namely, the viscose process and the cuprammonium process.
The viscose process comprises reacting cellulose with carbon disulfide in the presence of sodium hydroxide to obtain sodium cellulose xanthate, dissolving the sodium cellulose xanthate in an aqueous sodium hydroxide solution to obtain a cellulose dope (viscose), and coagulating the cellulose dope by contacting the cellulose dope with an aqueous acidic solution to thereby obtain a regenerated cellulose shaped article.
On the other hand, the cuprammonium process comprises dissolving cellulose in a cuprammonium solution to thereby obtain a cellulose dope, and contacting the cellulose dope with water and then, with an aqueous acidic solution to coagulate the cellulose dope, thereby obtaining a regenerated cellulose shaped article.
However, both of these two processes need a large amount of a poisonous substance, such as ammonia, carbon disulfide or a heavy metal (copper), in the step for preparing the cellulose dope or for shaping the cellulose dope. Therefore, the environment in the workshop for performing these processes is inevitably polluted.
Moreover, needless to say, these processes necessitate a step for recovering a poisonous substance. If the recovery step is incomplete, a large amount of the poisonous substance is inevitably caused to be discharged in the environment. Therefore, these processes are also disadvantageous from the viewpoint of protection of the environment.
Furthermore, these processes are disadvantageous in that these processes cannot be employed for producing an edible cellulose article.
In order to solve the problems of these processes, various proposals have been made and reported with respect to the environment friendly methods for producing regenerated cellulose shaped articles, which do not use a poisonous substance. As an example of such methods, there can be mentioned a method in which cellulose is dissolved in an aqueous alkali solution, such as an aqueous sodium hydroxide solution, to obtain a cellulose dope, and the obtained cellulose dope is used for producing a regenerated cellulose shaped article.
For easy understanding of the techniques for producing a regenerated cellulose shaped article by using a cellulose dope, abbreviations used for indicating the characteristics of cellulose and a cellulose dope are enumerated below.                DPc: viscosity average degree of polymerization of cellulose.        Cc: cellulose content (% by weight) of a cellulose dope, that is, the ratio of the weight of the cellulose contained in the cellulose dope (i.e., the sum of the weight of the cellulose dissolved in the cellulose dope and the weight of the cellulose remaining undissolved in the cellulose dope) to the weight of the cellulose dope.        CNa: sodium hydroxide concentration (% by weight) of a cellulose dope.        Sc: dissolution ratio (% by weight) of cellulose in a cellulose dope, that is, the ratio of the weight of the cellulose dissolved in the cellulose dope to the weight of the cellulose contained in the cellulose dope (i.e., the sum of the weight of the cellulose dissolved in the cellulose dope and the weight of the cellulose remaining undissolved in the cellulose dope).        Rc: amount of undissolved cellulose in a cellulose dope, more specifically, the volume (ml) of the undissolved cellulose contained in 20 liters of the cellulose dope.        
An explanation is made below with respect to specific examples of the above-mentioned environment friendly methods. Unexamined Japanese Patent Application Laid-Open Specification Nos. 62-240328 and 62-240329 disclose a method in which cellulose is subjected to steam explosion treatment to render the cellulose soluble in an aqueous alkali solution, the obtained cellulose is dissolved in an alkali solution to thereby obtain a cellulose dope, and the cellulose dope is subjected to wet shaping. The cellulose dope obtained and shaped in this method has a cellulose content (Cc) of only about 5% by weight.
In the above-mentioned method, any of poisonous substances, such as carbon disulfide, a heavy metal and an organic solvent, is not used. However, in the above-mentioned method, the solubilization of the cellulose is unsatisfactory.
In the process for producing a fiber, a film or the like from a cellulose dope, when the cellulose dope used has a Cc value of 5% by weight or more, it is required that the dissolution ratio (Sc) of the cellulose in the dope be 99.0% by weight or more.
When the cellulose dope has a Cc value of 5% by weight and an Sc value of 99.0% by weight, the cellulose dope has an undissolved cellulose amount (Rc) of about 7.9 ml. In the above-mentioned process, a filter is used in order to remove the undissolved cellulose contained in the cellulose dope. When the cellulose dope has a high Rc value, the filter is likely to be clogged. Specifically, when the cellulose dope has an Rc value higher than about 7.9 ml (which is an Rc value obtained when, for example, the cellulose dope has a Cc value of 5% by weight and has an Sc value lower than 99.0% by weight), the filter is frequently clogged, so that a commercial scale production of a fiber, a film or the like becomes practically impossible.
In an actual process for producing a fiber, a film or the like from a cellulose dope, it is required that the Rc value of the cellulose dope be lowered to about 1.6 ml by increasing the Sc value of the cellulose dope to about 99.8% by weight.
In view of this requirement, the cellulose dope obtained by the methods disclosed in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification Nos. 62-240328 and 62-240329 cannot be used in an actual process for producing a fiber, a film or the like, because the cellulose dopes obtained in these methods have a cellulose dissolution ratio (Sc) of less than 99.0% by weight and, hence, have an undissolved cellulose amount (Rc) as large as 10 ml or more.
As a method which solves the problem accompanying the techniques of the above patent documents, a method is proposed in which, in an attempt to solubilize the cellulose to a satisfactory level (i.e., to increase the Sc value) to thereby lower the Rc value of the cellulose dope, a slurry comprising cellulose dispersed in an aqueous medium is prepared, and a cellulose dope is produced by using the obtained slurry.
For example, Unexamined Japanese Patent Application Laid-Open Specification No. 9-316101 discloses the following two methods.
1) Cellulose is added to a first aqueous alkali solution having a low alkali concentration, wherein the particle size of the cellulose has not been adjusted and the viscosity average degree of polymerization (DPc) of the cellulose has been adjusted to 750 or less. The resultant mixture is stirred to disperse the cellulose in the first aqueous alkali solution, thereby obtaining a slurry. To the obtained slurry is added a second aqueous alkali solution having a high alkali concentration, followed by stirring at a high speed to dissolve the cellulose in the aqueous alkali solution, thereby obtaining a cellulose dope, wherein the second aqueous alkali solution is used such that the final alkali concentration of the cellulose dope becomes 7 to 11% by weight.2) Cellulose is pulverized in water such that the average particle diameter of the cellulose becomes 20 μm or less, thereby obtaining a slurry. To the obtained slurry is added an aqueous alkali solution having a high alkali concentration, followed by stirring at a high speed to dissolve the cellulose in the aqueous alkali solution, thereby obtaining a cellulose dope, wherein the aqueous alkali solution is used such that the final alkali concentration of the cellulose dope becomes 7 to 11% by weight.
The above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 9-316101 describes that, by the above-mentioned two methods, it is possible to produce cellulose dopes having a cellulose content (Cc) of 2% by weight or more and a cellulose dissolution ratio (Sc) of 99% by weight or more.
Specifically, the cellulose dopes obtained in the above-mentioned patent document include cellulose dopes which respectively contain 8% by weight of cellulose having a DPc value of 230, 5% by weight of cellulose having a DPc value of 350 and 2% by weight of cellulose having a DPc value of 750. These cellulose dopes have a cellulose dissolution ratio (Sc) of 99% by weight or more.
However, in the methods of the above patent document, if cellulose having a DPc value higher than that of the cellulose used in the above patent document is used or if a cellulose dope having a Cc value higher than that of the cellulose dope produced in the above patent document is produced (for example, if a cellulose dope containing 10% by weight of the cellulose having a DPc value of 230 is produced), the produced cellulose dope has a problem in that the cellulose is not satisfactorily dissolved in the aqueous alkali solution or in that the cellulose dope is very unstable and susceptible to gelation. Needless to say, such a cellulose dope susceptible to gelation cannot be used in the commercial scale production of a regenerated cellulose shaped article. As seen from the above, in the above-mentioned methods, at least one of the Cc value of the cellulose dope and the DPc value of the cellulose cannot be satisfactorily increased.
Generally, it is considered that the gelation of a cellulose dope occurs due to the association of molecular chains of cellulose, which association of the molecular chains is formed through the hydrogen bonds. (The association of the molecular chains of cellulose is one of the inherent characteristics of cellulose.) Specifically, it is considered that a plurality of molecular chains of the cellulose are associated with each other, wherein unsatisfactorily dissolved portions of the cellulose molecular chains, which portions are not satisfactorily solvated in the aqueous alkali solution, serve as cores of the association, so that the cellulose dope as a whole loses its fluidity, leading to a gelation of the cellulose dope.
By a method generally employed in the art, it is difficult to produce a cellulose shaped article having a strength sufficient for practical use from a cellulose dope containing cellulose having a low DPc value or a cellulose dope having a low Cc value. For obtaining a cellulose shaped article having a strength sufficient for practical use from such a cellulose dope, it is required to coagulate the cellulose dope in a special medium having a very high dehydration ability, such as a concentrated aqueous sulfuric acid solution having a concentration as high as 50 to 80% by weight.
Further, when a cellulose dope containing cellulose having a low DPc value or a cellulose dope having a low Cc value is used for producing a cellulose shaped article, it is very difficult to produce a desired cellulose shaped article (especially, a fiber or a film) by a method generally employed in the art and, hence, it becomes necessary to conduct the production of a cellulose shaped article by a special method. As an example of such special method, there can be mentioned a method in which, in a process for producing a fiber or a film from a cellulose dope, which comprises coagulating a cellulose dope, a net is used in an early stage of the coagulation (see Unexamined Japanese Patent Application Laid-Open Specification No. 7-278941). The reason for the use of a net is as follows. The fiber or film during the course of the coagulation has an unsatisfactory mechanical strength such that it is impossible to reel up the fiber or film by a generally employed process. Therefore, a net is used for reinforcing the fiber or film during the course of the coagulation. However, when a net is used, the operations involved in the production become cumbersome and the production efficiency becomes disadvantageously low.
In addition, as in the case of the production of a fiber or a film, the use of the cellulose dope containing cellulose having a low DPc value or the cellulose dope having a low Cc value in the production of the above-mentioned edible cellulose article is also accompanied by the problem caused by the poor mechanical strength of a cellulose shaped article (i.e., edible cellulose article) during the course of the coagulation. Therefore, the shape of the cellulose shaped article produced is inevitably limited. For example, in the case of the production of an edible cellulose article in the form of a fiber, the diameter of the edible cellulose article needs to be large; and, in the case of the production of an edible cellulose article in the form of a film, the thickness of the edible cellulose article needs to be large. Further, the blending ratio of a polysaccharide, a polypeptide or a natural resin in the edible cellulose article needs to be low. Therefore, the characteristics (such as palatableness) of the edible cellulose article cannot be satisfactorily improved.
With respect to the slurry comprising cellulose dispersed in an aqueous medium, many other examples are known. For example, such a slurry is disclosed in U.S. Pat. No. 5,123,962. However, the slurry described in this patent document is intended to be used as an edible additive, and is not intended to be used in a process in which a cellulose dope is produced from the slurry and the obtained cellulose dope is used for producing a general cellulose shaped article. Further, the cellulose used for producing the slurry has a DPc value of about 200 or less, so that, at least by an ordinary method, it is impossible to produce from the slurry a cellulose shaped article having a strength sufficient for practical use.
Descriptions on slurries used for producing a cellulose dope are also found in patent documents other than the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification Nos. 62-240328, 62-240329 and 9-316101. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 9-124702 discloses a technique for rendering cellulose soluble in an aqueous alkali solution by microfibrillating cellulose. In the technique of this patent document, cellulose is subjected to wet pulverization treatment using a stone mill type wet pulverizer to obtain a slurry comprising a microfibrillated cellulose dispersed in an aqueous medium, in which the number of microfibrils having a fiber diameter of 1 μm or less amounts to 95% or more, based on the total number of microfibrils (in a preferred mode, the number of microfibrils having a fiber diameter of 500 nm or less amounts to 90% or more, based on the total number of microfibrils).
This patent document also describes that, when a cellulose dope having a cellulose content (Cc) of 5.4% by weight is produced by using a slurry obtained by subjecting cellulose having a DPc value of about 700 to the above-mentioned treatment, and an aqueous sodium hydroxide solution having a sodium hydroxide concentration of 9.2% by weight, the produced cellulose dope has a cellulose dissolution ratio (Sc) of about 95%.
Further, the above patent document also describes that, when a cellulose dope having a cellulose content (Cc) of 7% by weight is produced by using a slurry obtained by subjecting to the above-mentioned treatment cellulose having a DPc value of about 300 (crystal type: cellulose II) which is produced from an alkali cellulose for use in the production of a rayon, and an aqueous sodium hydroxide solution having a sodium hydroxide concentration of 8.5% by weight, the produced cellulose dope has a cellulose dissolution ratio (Sc) of about 90%.
However, as mentioned above, in the process for producing a fiber, a film or the like from a cellulose dope, when the cellulose dope used has a Cc value of 5% by weight or more and has an Sc value of less than 99.0% by weight, it is practically impossible to produce a fiber, a film or the like on a commercial scale. Therefore, the cellulose dope cannot be used in an actual process for producing a fiber, a film or the like unless an operation for removing a substantial amount of the undissolved cellulose from the cellulose dope is conducted.
Further, as mentioned above, in the technique of Unexamined Japanese Patent Application Laid-Open Specification No. 9-124702, cellulose is microfibrillated by using a stone mill type wet pulverizer in order to produce a slurry containing a microfibrillated cellulose. However, the produced slurry has an extremely high viscosity, so that, in the production of a cellulose dope by the use of the slurry, the slurry needs to be stirred under a very high shearing stress. As described below, the extremely high viscosity of the slurry is considered to be caused by a network of microfibrils formed by the interaction between microfibrils of the cellulose. Further, as the degree of polymerization of the cellulose increases, the yield shearing stress of the slurry increases, so that a high shearing stress is needed for stirring the slurry. Therefore, it is very difficult to produce a cellulose dope by using the above-mentioned slurry.
In general, as a pretreatment conducted prior to the production process of the above-mentioned viscose (i.e., cellulose dope obtained by dissolving sodium cellulose xanthate in an aqueous sodium hydroxide solution), the cellulose is pulverized in an aqueous alkali solution. As the aqueous alkali solution, an aqueous alkali solution having an alkali concentration as high as 10% by weight is generally used. It is considered that the aqueous alkali solution having such a high alkali concentration is used for the purpose of improving the pulverization efficiency. However, when cellulose is contacted with an aqueous alkali solution having such a high alkali concentration, the cellulose is converted into a crystalline alkali cellulose. Since a crystalline alkali cellulose is completely insoluble in an aqueous alkali solution, the slurry obtained by the above-mentioned method cannot be used for producing a cellulose dope containing an aqueous alkali solution as a solvent.
Unexamined Japanese Patent Application Laid-Open Specification No. 4-348130 discloses a method for producing particulate cellulose by aging cellulose in the presence of an alkali.
However, the cellulose used has a viscosity average degree of polymerization (DPc) of from 20 to 150. As already explained above, it is extremely difficult to produce a cellulose shaped article for practical use by the use of cellulose having such a low degree of polymerization. Moreover, it should be noted that the above-mentioned aging in itself is a technique for converting cellulose into a crystalline alkali cellulose which is completely insoluble in an aqueous alkali solution. Therefore, the technique of the above patent document cannot be used for the production of a cellulose dope containing an aqueous alkali solution as a solvent.
From the above, it is apparent that a conventional cellulose dope containing an aqueous alkali solution as a solvent for cellulose, which is produced by the conventional method, has various problems caused by the unsatisfactory solubilization of the cellulose. The problems can be enumerated as follows:
the cellulose dope contains a large amount of undissolved cellulose, so that the cellulose dope cannot be used in an actual production process;
the amount of cellulose having a satisfactory degree of polymerization, which is dissolved in the cellulose dope, is not sufficient for producing a cellulose shaped article having a sufficient strength for practical use; and
the cellulose dope is unstable and susceptible to gelation.
Thus, the conventional cellulose dope has a problem in that there are fatal defects with respect to the mechanical properties of the cellulose shaped article produced from the conventional cellulose dope and the procedure necessary for producing the cellulose shaped article from the conventional cellulose dope.